Arthropoda, a phylum within Kingdom Animalia, is the most diverse of all the phyla. Fossil records indicate that arthropods evolved from annelids sometime in the Cambrian period, but molecular evidence indicates that they evolved separately from each other.

All arthropods are bilaterally symmetrical, and their bodies are covered with a tough exoskeleton. This exoskeleton consists of layers of chitin (a type of polysaccharide) and proteins. This hard covering protects the animal, and provides points of attachment for muscle cells, but also present a unique problem for the animal during growth: because it (unlike the human epidermis) is not living, it cannot grow with the organism. Once the arthropod has grown too large for its exoskeleton, it is shed and a new skeleton is grown. This entire process is known as molting.

Arthropods' bodies are segmented, but they are also divided into distinct sections, usually the head, thorax, and abdomen, although the head and thorax can be joined to form a cephalothorax.

There are five main groups of arthropods: horseshoe crabs, arachnids, crustaceans, centipedes and millipedes, and insects. Horseshoe crabs are unique in that they have survived for hundreds of millions of years with almost no evolution. Arachnids consist of scorpions, spiders, and mites. Crustaceans are nearly all aquatic and consist of lobsters, crabs, shrimps, barnacles, etc. Centipedes and millipedes are both worm-like, segmented animals with a single pair of legs on each body segment.

Overall, Arthropoda is the most successful phylum in terms of diversity, geographical distribution, and sheer numbers. Most strikingly, perhaps, the total population of arthropods on Earth is estimated at around one billion billion individuals!

Phylum Arthropoda includes the following classes:

• Class Crustacea (The crustaceans)
  
• Class Uniramia, also known as Insecta (The insects, millipedes, and centipedes)
  
• Class Trilobita (extinct)
  
• Class Cheliceramorpha (The horseshoe crabs and arachnids)